An assist device that assists user's motion by applying torque to joints has been developed. Among such assist devices, a device that reinforces muscle power for a healthy person may be commonly called a “powered exoskeleton”. A device that assists muscle power for a user whose muscle power has declined or a user who has difficulty to move joints on his/her own may be commonly called as a “motion assist device”. With regards to the motion assist device, great numbers of researches on assisting leg muscle power, especially walking motion, are being made. A device that assists the leg muscle power is referred to as a “leg assist device” herein.
The leg assist device mainly assists the muscle for moving a knee joint. Such device typically has a mechanical configuration in which an upper leg link to be attached to the user's upper leg and a lower leg link to be attached to the user's lower leg are connected. The upper leg link and the lower leg link are connected by a rotary joint with an actuator. The device guides swing of the user's lower leg, i.e., knee joint motion, by driving the lower leg link. One example of the leg assist device having the above mechanical configuration is disclosed in Japan patent application publication No. 2008-006076.
The leg assist device having the above mechanical configuration can assist walking motion, standing up motion, and seating motion by changing a control law for the actuator. The technique disclosed herein provides a leg assist device with a control law for assisting standing up motion. One example of the device assisting standing up motion is disclosed in Japan patent application publication No. 2009-060946.